In a large pot of boiling salted water, cook pasta according
to package instructions. Reserve 1/2 cup pasta water; drain pasta and
return to pot. Meanwhile, in a large nonstick skillet, cook bacon over
medium until fat is rendered and bacon is crisp, 10 minutes. With a
slotted spoon, transfer bacon to paper towels to drain. Add butter,
onions, and garlic to skillet and cook until onions are softened, 10
minutes. Add cauliflower, cover, and cook, stirring occasionally, until
cauliflower is tender, 12 minutes. Stir in bacon and sage and cook until
fragrant, 1 minute. Add cauliflower mixture to pot with pasta. Stir in
vinegar and enough pasta water to create a thin sauce that coats pasta.
Season with salt and pepper.